Site Description Bryn Celli Ddu is a Late Neolithic passage grave in the European Atlantic tradition, excavated and partly restored in the mid to late 1920s by W J Hemp (Hemp 1930). It comprises an outer circular stone kerb c. 26m diameter, with an inner stone arc, both of which encircle a simple passage tomb whose entrance lies on the east side. Hemps' original hypothesis, that the tomb was built within a ruined henge, is nowadays seen as problematic.

The passage tomb is one of the finest of its kind in Wales. The c.7m long inturned forecourt and stone-lined entrance passage gives access to a central polygonal chamber made of large slabs. In the north angle of the chamber is a 1.7m high smoothed stone pillar, interpreted as a 'protectress' or tomb guardian in the style of Breton tombs, or a phallic symbol. One of the chamber stones bears a small spiral carving which is probably Neolithic. A solar alignment on midsummer sunrise, first postulated by Sir Norman Lockyer in 1909, was finally proven and documented by Dr Steve Burrow of the National Museum Wales in 2005. A central pit contained the most richly decorated Neolithic carved stone in Wales. The original is in the National Museum Wales, with a cast on site.

Bryn Celli Ddu sits at the heart of a ritual landscape, with a plough-levelled cairn just to the south (NPRN 309540), a standing stone to the south-west (NPRN 302503) and a cup-marked rock to the west (NPRN 415847). The arrangement of the passage tomb and style of the carvings has similarities with the passage tomb of Barclodiad y Gawres on western Anglesey (NPRN 95545).